The invention comprises a method for predictively assessing one or more characteristics of wood fibre or wood pulp produced from wood while the wood is in solid form, such as fibre length or the strength of pulp formed from the wood. The invention has particular application in the selection of wood for pulp and paper or other wood-fibre-based applications such as the production of fibre board.
Optimum segregation of tree stems after felling, or of logs after sawing of the stems into logs, for different applications such as solid or structural grade lumber, manufacture of reconstituted wood products, or pulp and paper manufacture, is an important issue for the forestry and wood products industries because of the variable nature of the raw material and the different properties required in different end products. There are major commercial benefits to be gained by optimising the use of the wood resource for different solid wood, reconstituted, and pulp and paper applications, which may require different properties in the raw material. Differences in the raw material arise due to genetic differences, silvicultural differences, and geographical and site differences. Even within a tree there are differences in wood properties between the corewood of the tree and the outer wood, and also variations from the bottom to the top of the tree, which further complicates the wood segregation issue.
At present, historical wood density information and a general knowledge of trends in fibre properties in different parts of the tree are the only tools available to perform some limited segregation of wood for pulp and paper manufacture for example. Basic density is difficult to measure, and it is correlated with fibre properties only at a population level and so cannot be applied at an individual tree or log level. Little information on wood quality at an individual stand level is available, other than average wood density data from historical measurements, and there is generally no information at all at an individual tree level. To date, basic wood density is the only tool available. Most wood properties are related to the basic density of the wood (ie the amount of dry wood substance per unit volume of wood), but basic density is difficult to measure and impossible to usefully apply at a whole tree level. Density is also a measure of the amount of wood substance and void space in a unit volume of wood, and is not indicative of the number of fibres which make up that space. For example, individual trees can have roughly the same chip basic density, but have very different fibre lengths, fibre width and thickness, fibre wall areas (coarseness), and number of fibres per unit mass of wood.
Some wood segregation on the basis of density is practised in pulp and paper manufacture. High density wood from the outside of the tree (the slabwood) generally has long and coarse (thicker-walled) fibres which are suitable for certain products such as cement-board reinforcing pulp and sack grades. Wood from the top of the tree is more like corewood and has lower density with shorter, thinner walled (low coarseness) fibres. Pulp from this wood is more suitable for printing and writing or tissue grade papers for example. However basic density is still a very crude basis for wood segregation and knowledge of density variability is rather poor.
Softwood kraft pulp qualities are normally determined by the handsheet properties of apparent density or bulk, tensile strength, and out-of-plane tear strength. Fibre length is the critical softwood kraft pulp fibre determinant. With too little fibre length such pulps lose their characteristic softwood reinforcement properties. However, very long, coarse fibres can be difficult to refine and are prone to flocculation and sheet formation becomes a problem. Other fibre properties are also important but only after fibre length requirements are met. Fibre length is critical for the reinforcement properties of softwood kraft pulps. If pulp fibre length falls below a certain critical level of about 2.0-2.1 mm, bulk is abruptly decreased and reinforcement tear/tensile strengths are correspondingly, abruptly lowered.
The Wet Zero Span Tensile (WZST) strength of a pulp is influenced by the number of fibres per unit mass of pulp and by the strength of the individual fibres. Individual fibre strength is in turn influenced by fibre coarseness and intra-fibrewall characteristics such as micro-fibril angle (MFA). WZST strength is a good predictor of Tear Index at a given Tensile Index, the traditional indicator of softwood kraft reinforcement potential.
Pulp quality is very much a function of its end-use. As well as the above mentioned quality factors, end-users of market pulps are also concerned about: ease of beating the amount of energy required to refine a pulp to an acceptable tensile strength; reinforcement potential; and effects on paper sheet formation.
Pulp and paper mills normally utilise the residues from harvesting such as top logs and low grade logs that arise during harvesting operations, and from saw milling such as chips from the slabwood sawn from the outsides of the log.
The measurement of velocity of sound in a log is a non-destructive technique which can be used to evaluate the stiffness of materials by means of sound transmission. The sound wave, for instance induced by the impact of a hammer at one end, travels down the length of a log. The transit time (xcex94t) is measured. The modulus of elasticity (MOE) is computed from the transit time and density (p) as follows:
MOE=V2p=(l/xcex94t)2p
This is a fundamental relationship for materials. Although a log of wood is not a homogeneous material (compared with an iron bar, for example), and does not obey this law perfectly, relatively good relationships have been found between sound wave speed and the average measured stiffness of lumber which is sawn from the log. U.S. Pat. No. 4,144,669 describes the use of velocity of sound measurement for the grading of wood. Measurements of velocity of sound in a log can be made with industrial stress wave timers, which are commercially available.
In broad terms in one aspect the invention comprises a method for predictively assessing one or more characteristics of wood fibre or wood pulp produced from solid wood, comprising determining the velocity of sound through the solid wood, and assessing characteristic(s) of wood fibre or wood pulp produced from the wood by reference to the velocity of sound through the solid wood.
More particularly the method comprises causing a sound wave to be transmitted through the wood, determining the velocity of the sound wave through the wood, and comparing the result to stored information on fibre characteristic(s) versus sound velocity through the wood-type to determine the fibre characteristic(s) for the wood.
In one form the method includes placing a sensing means capable of detecting sound in the wood in contact with or within sensing distance of one end of the length of wood, placing a second sensing means capable of detecting sound in the wood in contact with or within sensing distance of another end of the length of wood, causing a sound wave to be transmitted in the length of wood from one end to the other, detecting the sound at each end of the log or length of wood, determining the velocity of the sound in the wood, and assessing characteristic(s) of wood fibre or wood pulp produced from the wood by reference to stored information on fibre characteristics versus sound velocity through the wood.
In another form the method includes placing means capable of detecting both an original and reflected sound wave in contact with or within sensing distance of one end of the length of wood, causing a sound wave to be transmitted in the length of wood, detecting a reflected echo of the sound in the wood, determining the velocity of the sound in the wood, and assessing characteristics of wood fibre or wood pulp produced from the wood by reference to stored information on fibre characteristics versus sound velocity through the wood.
In broad terms in another aspect the invention comprises apparatus for predictively assessing one or more characteristics of wood fibre or wood pulp produced from solid wood, comprising sensing means capable of detecting the velocity of sound in through the wood, and computer processing means comprising stored information on fibre characteristics versus sound velocity in wood and arranged to determine the fibre characteristic(s) by reference to said stored information on fibre characteristic(s) versus velocity through the wood.
By xe2x80x9csoundxe2x80x9d is meant normally relatively low frequency energy that will be audible to the human ear eg of the order of 500 to 1000 Hz as will be created by a single impact on a length of wood, by striking the length of wood with a hammer for example. Higher frequencies may be used such as of the order of 15 kHz or above including ultrasound but use of lower frequencies is preferred.
The method and apparatus of the invention may be used with whole stems after felling, logs and sawing of the stems into logs, flitches, cents, or other lengths of lumber or wood in any form, and in this specification and claims xe2x80x9clength of woodxe2x80x9d is to be understood accordingly.